Various types of fluid flow control valves are used in applications where the valve is subjected to corrosive acidic or caustic liquids or where the purity of the liquids which flow through the valve must be maintained. Such valves are constructed of relatively inert materials, e.g., fluoropolymers or other polymeric materials, or the valve surfaces which come into contact with the flowing liquids, or which potentially can come into contact with the liquids, are coated with inert materials.
Such flow control valves are commonly biased closed by a spring force and are opened by means of a solenoid actuator, or by pneumatic or hydraulic pressure, or the like. When valve closure springs are used, it can be important that the force required to be exerted by the spring to close the valve is minimized. By minimizing the required spring force, the spring imparts less stress to the valve structures, thereby increasing the length of valve life.
Fluid control valves sometimes include a diaphragm which is in contact with the fluid and which provides a barrier against escape of the fluid into the valve operating mechanism or into the atmosphere. In some designs, a backup diaphragm is provided which, in combination with the barrier diaphragm, provides a chamber to contain any fluid which may leak through the barrier diaphragm. Such chambers are sometimes provided with leak ports, so that, if the barrier diaphragm fails, the fluid which passes through the diaphragm into the chamber will be detected and appropriate action can be taken.
For example, U.S. Pat. No. 4,010,769 discloses a valve which incorporates a barrier diaphragm which contacts the fluid which flows through the valve. A second diaphragm is in the valve above the first diaphragm, and a leak port is between the diaphragms. Any fluid which may leak through the barrier diaphragm will be detected by means of the leak port and appropriate corrective action can be taken.
When a valve incorporates a diaphragm which contacts the fluid in a system, and the diaphragm is connected to the valve operating mechanism, as is the case with the valve disclosed in the '769 patent, any force which the fluid exerts on the diaphragm is transmitted to the valve operating mechanism and, thus, affects the operation of the valve. For example, in the '769 valve, a spring biases the valve closed and the valve opens by means of a solenoid. Any force exerted on the barrier diaphragm by the fluid in the system will tend to open the valve. Therefore, a larger spring is required to hold the valve closed than would be necessary if the valve was designed so that the force on the diaphragm tending to open the valve would be counterbalanced by another force in the opposite direction. The use of a larger spring results in more stresses than necessary being imparted on the valve structure, thereby tending to reduce the length of the life of the valve.
When a valve is provided with a diaphragm that is attached to the valve's operating mechanism which therefore moves with the mechanism as the valve opens and closes, the diaphragm can fail due to fatigue cracking. Usually, the greater the distance the diaphragm moves, and the more stretch and strain that is applied to the diaphragm during each cycle, the fewer cycles the diaphragm will be able to withstand before failing. Thus, it is important to minimize the length of diaphragm travel, and the strain on the diaphragm, to increase valve life.
It is therefore desirable to provide to the art a flow control valve for use with either chemically pure or corrosive liquids which uses a minimum spring force for biasing the valve and which is designed to reduce the distance the diaphragm must travel and the strain on the diaphragm so that the operation life of the valve is enhanced.